Dow Corning installed a new two-stage vacuum pump in one of the processing plants in its chemical manufacturing complex in Midland, Michigan, where the pump was needed to draw gases from a process reactor vessel. The pump was put into operation using a conventional mineral oil that was specified by the original equipment manufacturer. During the first nine months of operation there was a catastrophic failure occurring every four to six weeks.

“There was so much corrosion that it caused the casing to pit, and we couldn’t just make repairs,” said Lisa Honaman, building engineer. “We had to order a new pump casing and other major parts every time, then spend one to two days rebuilding the pump.”

Each breakdown was estimated to cost $8,000 to $9,000. The repair parts cost $6,400 and labor averaged $2,000, if the work could be scheduled during regular hours.

High-temperature Additive Reaction
The pump drew hydrochloric acid vapors from a reactor vessel used to manufacture pharmaceutical “actives.” During the vacuum process, some of the acidic vapor condensed into a liquid form and mixed with oil inside the chamber. As the oil stream hit the inside of the chamber casing, the zinc-based antiwear additives in the conventional oil mixed with the hydrochloric vapors and catalyzed a reaction drawing away metal from the casing and creating ferric chloride. The result was a pitting action in which metal particles flaked off of the inside casing of the vacuum chamber, where the tips of the rotor vanes must provide an oil and vacuum seal. Cracks in the chamber housing revealed further deterioration.

The pitting created imperfections that corrupted the full integrity of the surface of the inside casing and the tips of the rotor vanes. These imperfections prevented the formation of a reliable oil film needed to maintain a vacuum seal while preventing surface contact to eliminate wear.

Combined with the acidic oil, the resulting increase in a higher operating temperature contributed to a breakdown of the phenolic/fiber composition of the rotor vanes. As the rotor turned, particles chipped off the trailing edge of the vanes and damaged the tips of the following vanes, which compounded the damage through increased vacuum leaks, pitting and overheating. This lead to total vane breakdown, extensive case damage and catastrophic failure.

All of the signs pointed to a high-temperature additive reaction as the problem.

Low-additive Chemistry is Key to Success
Dow Corning’s Technical Services staff suspected that a more chemically inert lubricant would result in better performance by eliminating or changing the effects of the additives reacting with the process gases, and attacking metal pump components. As a result, they recommended a low-additive mineral oil. The solution was Dow Corning’s Molykote® L-0610 vacuum pump oil.

It is an ultra high-purity mineral oil based on new, low-additive technology chemistry that minimizes the opportunity for chemicals to react to process gases. Its proprietary additives package gives the oil its ability to reduce oxidation rates caused by elevated pressures and temperatures that can also cause premature lubricant breakdown.

Traditional solvent-refined oils with zinc-based antiwear chemistry typically have a higher initial acid number. “Add water and hydrogen, and you could make sulfuric acid,” said Honaman.

Mineral oils with zinc-free chemistry have a lower initial acid number. With less than five parts per million sulfur, its rust and oxidation preventive additives are virtually free of sulfur.

Smooth Operation
After plant maintenance professionals switched to the low-additive mineral oil formulation and installed a larger oil pump, there was a 30 percent drop in surface operating temperature. While the exact cause of the temperature reduction was not known, both steps were believed to be contributing factors. The key problem solved, however, was the elimination of metal failure caused by process gas -additive chemistry reaction.

Since switching oils, the vacuum pump has been operating smoothly with no breakdowns. As a result, plant personnel switched oil in all of the vacuum pumps, reducing the number of lubricants by almost 50 percent, from 13 products to six.

Reducing the number of commercial lubricants provides additional efficiency improvements by simplifying the purchasing process and reducing the potential for applying the wrong lubricants.